An important environmental and economic consideration with respect to fluid transmission pipeline operation and maintenance pertains to the monitoring of the pipeline from time to time to determine any changes in curvature which might result from earth heaving or subsidence. Changes in pipe curvature from a predetermined course may indicate impending failure which could be economically and environmentally catastrophic. Moreover, many fluid transmission pipelines have been laid with inaccurate or only generalized surveys of their location or course and it is important to determine with some accuracy the location of a pipeline at a given time for various reasons, including subsequent monitoring of the pipeline to determine if any changes in location or curvature have occurred which require attention or remedy.
The development of so called inertial navigation systems for aircraft and other vehicles has provided some background for utilizing the capability of this kind of equipment in pipeline monitoring and other survey processes. For example, U.S. Pat. No. 4,524,526 to S. Levine describes an apparatus for detecting deflections along the length of a pipeline wherein an inertial reference system is placed onboard a body which may be launched into and traversed through a pipeline, said body being commonly known in the pipeline art as a pig. The inertial refernce system of the Levine patent generates signals indicative of its position in space which must be corrected by measuring any changes in the position of the pipeline pig with respect to the piepline in order to arrive at the actual position or change in curvature of the pipeline itself. The device of the Levine patent requires a specially constructed pipeline pig having support arms which include sensing devices for measuring the change in attitude of the pipeline pig with respect to the pipeline so that any erors measured by the inertial reference system can be corrected to determine the actual location or changes in curvature of the pipeline.
Another approach to providing an improved pipeline survey or curvature measurement system is described in U.S. patent application Ser. No. 831,228 filed Feb. 19, 1986, in the name of Pedro F. Lara and assigned to the assignee of the present invention and now U.S. Patent No. 4,717,815. The system described in the abovementioned patent application utilizes an inertial measurement unit comprising three orthogonally mounted accelerometers which are mounted within a pipeline pig and arranged along the central axis of the pig for taking measurements of changes in pipeline curvature and longitudinal position of the pig in the section of pipeline being measured. Changes of position such as roll and pitch attitude of the pig within the pipeline are minimized by providing for the center of gravity of the pig to be placed laterally spaced from the longitudinal pig axis, which axis is designed to be coincident with the pipeline axis by virtue of the construction of the pig support structure. Alternatively, the measurement system described in the subject patent application utilizes a clinometer to correct for accelerometer signals resulting from any rolling of the measurement unit about its axis within the pipeline during traversal thereof.
In the further development of pipeline survey and curvature measurement systems, it has been determined that it is particularly desirable to provide a structure for supporting and carrying an inertial measurement unit which undergoes minimal excursion relative to the pipe axis and to provide an inertial measurement unit and associated signal computing and recording apparatus which does not require compensation for movement of the support structure relative to the pipeline itself. This elimination of a potential source of error in making pipeline curvature and survey measurements is particularly important taking into consideration numerous factors. One important factor in making pipeline curvature and survey measurements pertains to the need to utilize an instrument support body, such as a pipeline pig, which is somewhat conventional in construction in the sense that the pig may be accommodated by a pipeline including its launching and receiving fittings, valves and other devices normally found within the pipeline with minimal likelihood that the pig will become stuck in the line during traversal thereof. For apparent reasons, it is important that the reliability factor be very high with regard to being able to traverse a survey device through an existing pipeline with minimal chance of damage to the pipeline or stalling of the device within the line. Moreover, the accuracy of the measurements taken by certain types of inertial measurement units onboard a pipeline pig also requires special considerations in construction the pig which have actually resulted in a new and inventive concept and approach to providing a pipeline curvature measurement and survey system.